"The Quantitative Evaluation of the Population Kinetics of Tumors in vivo and the Effects of Radiation and Drugs upon them" is a long range project aimed at a more quantitative understanding of the effects of therapeutic procedures on tumor growth or regression. Transplantable mouse leukemias are precisely followed in vivo by labeling their DNA with radioiodine incorporated in iododeoxyuridine, observing both the extent of cell death and metastatic infiltration, which varies widely among the several cell lines studied: L1210, L1210/MTX, L5178Y, Gardiner lymphoma, Ehrlich ascites, and rat WR-6. Cell killing by x- rays, by BCNU, cytoxan amethopterin, hydroxyurea, cytosin arabinoside, and asparaginase can be measured precisely if one allows for effects of the agents on the host as well as on the tumor. I125 when incorporated into tumor cells is 20 times as toxic as I131. I131 labeling permits the administration of small inocula and the study of immune responses. In the next period, we will continue to investigate host influences on these processes. This will involve study of the host immune response to tumors plus the "natural resistance" we are observing in non-immune recipients. We also want to understand differences in host resistance which depend on tumor location. We have developed a radio-immunoassay for serum thymidine and find that serum thymidine levels can become markedly elevated following irradiation or in advanced tumors.